This invention relates to a cogeneration system where a gas water heater, furnace, or the like includes photovoltaic means for generating electricity for powering its own blowers, ignition devices, and the like, and even including generating more power than required for its own operation for use in other devices.
Gas-fired water heaters are rarely provided with electric power. Thus they are essentially gravity devices with convective circulation of combustion products and water. It would sometimes be desirable to augment circulation with electric blowers. It would also be desirable to have automatic electric ignition rather than a standing pilot for minimizing fuel consumption.
A completely gas water heater which generates its own electricity can replace electric water heaters in building situations in which gas water heaters must have power vents. These situations occur where the cost of installing vertical roof vents is high and the only means of venting is through an external wall. In such cases the cost of the power vent and associated vent safety systems may be prohibitive, and electric heaters are installed because the initial installation is less costly. With a self-powered gas-fired water heater the more economical gas fuel can be used.
Gas-fired furnaces for space heating are commonly connected to electric power for operating the system's blowers, and in more recent furnaces to provide intermittent ignition rather than having a standing pilot flame continuously burning. Overall fuel economy may be promoted by providing the blower with electric power cogenerated with the space heating provided by the furnace.
Thermocouples have long been used in gas-powered appliances for generating a small amount of electric current. Typically a thermocouple is placed in the pilot flame to generate just enough power to keep a fuel control valve open. This operates as a safety precaution so that the absence of power from the thermocouple cuts off the flow of fuel. There is insufficient power from such a thermocouple for opening such a valve, which is commonly reset manually, let alone operate a blower or auxiliary devices.
Electric power can also be generated by photovoltaic devices. U.S. Pat. No. 3,188,836 by Kniebes describes use of emissive radiation to generate power to control a valve for a gas lamp. This was, in effect, a replacement for a thermocouple.
Rather different technology involves use of photoelectric devices which change resistance, for example, when illuminated. These devices, in effect, act as switches for controlling current from sources of electric power. These systems are not self powered since the photoelectric devices do not generate electricity. Exemplary of use of photoelectric devices in appliance control can be seen in U.S. Pat. No. 2,306,073 by Werth.
U.S. Pat. No. 3,331,701 by Werth provides the first known description of a thermophotovoltaic power producing device using silicon cells. The efficiency of silicon solar cells has been optimized to produce electric power with an efficiency of about 26% using a tungsten filament heated to about 2200.degree. K. as the heat source. This would be no more than marginally suitable for a self-powered gas fired appliance as provided in practice of this invention.
It is therefore desirable to provide a highly efficient means for generating electric power in a gas-fired appliance so that blowers and other auxiliary electric devices can be operated entirely by power generated by the gas-fired appliance. Such cogeneration of electric power and a heated utility fluid, such as in a hot water heater or space heater, can provide high thermal efficiency.